Since people don't like to read long things, I'll make it short.
Brain Development and Effect on Intelligence:
Using marijuana as an adolescent is indeed not a smart idea. Not because it will harm you, but because most teenagers are not mature or responsible enough to handle the herb. When you grow up and can take care of your responsibilities and smoke at the end of the day, you deserve to. Right now, you're just going to fail in life and blame it on weed instead of your self.
According to recent studies there is no structural change associated with adolescent cannabis use. While this might seem weird, even the government is now admitting this fact. Even heavy marijuana usage will not damage your brain functions if you read studies, such as this one. There is truly no possibility for you to say marijuana damages an adults or adolescent's brain structure or function, in fact it's been shown to increase the IQ of both parties read this. Delta 9 Tetrahydrocannabinol protects hippocampal neurons from excitotoxicity
Can marijuana cause Schizophrenia?:
Marijuana usage is not associated with the development of Schizophrenia. Although, it has been shown to decrease the amount of time it will take to trigger in a person born with the mental illness. Without a pre-disposition, marijuana cannot cause you to develop schizophrenia or psychosis. Dutch scientists have recently shown this in a recent study conducted.
What is the impact on your lungs? Marijuana can cause mild lung problems such as wheezing or bronchitis with long term heavy usage, but this will go away with a few days of not smoking. This is due to marijuana's bronchial dilator effect, which dispels all of the gunk you have accumulated in your lungs. To clean it out, it does things like this.
"Findings from the present long-term, follow-up study of heavy, habitual marijuana smokers argue against the concept that continuing heavy use of marijuana is a significant risk factor for the development of [chronic lung disease]," concluded the UCLA study. "Neither the continuing nor the intermittent marijuana smokers exhibited any significantly different rates of decline in [lung function]" as compared with those individuals who never smoked marijuana. Researchers added: "No differences were noted between even quite heavy marijuana smoking and nonsmoking of marijuana." These findings starkly contrasted those experienced by tobacco-only smokers who suffered a significant rate of decline in lung function.
Marijuana and Acne:
Believe it or not, marijuana does indeed help get rid of your acne and fix skin conditions. In this newly discovered endocannabinoid system, it is shown that marijuana has many auto-immune modulatory effects and components of it are already present in our natural skin functions.
Does smoking marijuana lead to harder drugs? Is it addicting?
People for years have believed in the gateway theory, but recent studies of marijuana users show the opposite of this. Marijuana users tend to smoke cigarettes and drink alcohol even less than people who do not, according to recent studies. In reality, once people reach the age of 21, marijuana usage is shown to not lead to the usage of harder drugs such as cocaine, MDMA, LSD, etc... and in fact will lower the likelihood of the person using these drugs.
Marijuana has been shown to not have any physical withdrawals, but even so, in a person with a weak will they will likely experience psychological withdrawal. They became so used to marijuana aiding in their sleep, in their hunger, and their emotions - once they stop, they realize how hard life is. This leads to less hunger, a hard time falling asleep, and moodiness for the first few weeks. This happens to very few people, but it can still happen. This is only temporary, and you can over come it with your mental abilities if you have a strong will.
Is Cannabis Smoke Carcinogenic? Can it cause Cancer?:
Tobacco has dramatic negative consequences for those who smoke it. In addition to its high addiction potential , tobacco is causally associated with over 400,000 deaths yearly in the United States, and has a significant negative effect on health in general . More specifically, over 140,000 lung-related deaths in 2001 were attributed to tobacco smoke . Comparable consequences would naturally be expected from cannabis smoking since the burning of plant material in the form of cigarettes generates a large variety of compounds that possess numerous biological activities .
While cannabis smoke has been implicated in respiratory dysfunction, including the conversion of respiratory cells to what appears to be a pre-cancerous state , it has not been causally linked with tobacco related cancers  such as lung, colon or rectal cancers. Recently, Hashibe et al  carried out an epidemiological analysis of marijuana smoking and cancer. A connection between marijuana smoking and lung or colorectal cancer was not observed.
tl;dr Marijuana is not that harmful with the exception of the occasional wheezing, coughing, and bronchitis - all temporary.
If you would like to learn more, search for "Granny StormCrow's Medical Marijuana List 2012", it addresses one thousand topics I have not.